code: def data_frequency(self):
      return self._data_frequency
  
file_path: catalyst/catalyst/exchange/exchange_bcolz.py
imports: import numpy
  from catalyst import get_calendar
  from catalyst.data.minute_bars import BcolzMinuteBarReader, BcolzMinuteBarWriter
label_desc: Use image node_id 2 for calling the BcolzExchangeBarReader obj's underlying member method code with example usage: obj.data_frequency() and returns: self